Nitrogen and Nitric Oxides

[nmassi]

I have come across mixed information on what NOx is. Most online resources I am finding say that "NOx"
is the sum of NO and NO2. However, manufacturers that offer "NOx sensors", such as Hach and YSI, are measuring and taking the sum of NO2 and NO3. Is there a technicality I am missing where the sum of NO2 and NO3 is correct for measuring NOx? Does the sum of NO2 and NO3 yield a good indication of NOx? Or are these manufacturers using the chemical acronym "NOx" incorrectly?

[scooke · Accepted solution]

Generally, "NOx" means ANY combination of Nitrogen and Oxygen.  The "x" is just like the one used in maths - it is a variable placeholder designating a range of values.  "NOx" can be NO, NO2 (N2O4), NO3, or any combination of them.  NO3 does not really exist as a gas, but may be of interest in the atmosphere as HNO3 or HNO2.  Those are also the species usually found in water samples.

For simplicity, most detectors are designed to detect ALL NOx forms by converting NO to NO2 and detecting the NO2 total.  Detection mechanisms are a different matter, so some sensors may only detect certain forms of NOx.  The Hach and YSI analyzers are just for water samples and are really sensing the ionic forms, not the gases.  The Hach line has many different sensing modes and applications, including the UV Spectroscopy used in the YSI unit.

Best regards,

Steven